And clearly at 100 mph, your goal is getting somewhere faster - not maintaing fuel economy.
I think it's more about the ability or power to do 100MPH when you need it, than to actually do it.
If your car can do only 80MPH top speed, and you have to get somewhere at 80MPH, your car will not like this.
There's something like a 20-40% overengineered car, that gets the job done well.
The Elantra I drove had 50MPG at 75MPH instantaneous. My Cruze Eco on the other hand in the same situation, would do about 40-45MPG. With that I mean, the readout over several miles. Not just looking at the highest number, or releasing the throttle and taking a pic of the readout.
This week even I was pondering about the accuracy of the pump.
We all base our readouts at the pump, but do you know that pumping gas, there's a vent hose that will recycle gasoline vapors, and sometimes even gasoline, back in the tank?
We expect the pump to be calibrated, but a lot of things can cause an off reading.
Like small spills, fuel in the vapors being sucked back up, fuel pumps pumping air or other debris (I had a Marathon gas station that had small pebbles in the fuel); ...
And then we're bitching about the incorrect fuel readout of our cars?